Checking the linear box will make your image look very dim or dark, but what it does is makes sure that your image contains 100% of the dynamic range captured by the sensor. You'd use it for problem images with (say) extreme highlights and shadows to contend with at each end if the dynamic range, or simply to get the best range of tones possible out of a black and white image.
You then output the image 'as is' to your usual editing package for further adjustment of highlights, midtone, shadow, gamma, contrast etc. to get the best dynamic range out of the image.
It's probably most useful for black and white images or black and white conversions. You can use it for colour images, but it's far more difficult, as you get some very strange looking colours due to no tones curves being applied which would be the case normally, and that can be difficult to correct.
Definitely one to practice with to get the best from it.